Development of an approach for interface pressure measurement and analysis for study of sitting

DEVELOPMENT OF AN APPROACH FOR INTERFACE
PRESSURE MEASUREMENT AND ANALYSIS FOR
STUDY OF SITTING

Wu Yaqun

A THESIS SUBMITTED
FOR THE DEGREE OF MASTER OF ENGINEERING
DEPARTMENT OF MECHANICAL ENGINEERING
NATIONAL UNIVERSITY OF SINGAPORE
2010


Acknowledgements

Acknowledgements
I would like to express my deepest appreciation and gratitude to the following people
for their guidance and advice throughout the course of this project:


Prof Wong Yoke San, Supervisor, National University of Singapore,
Department of Mechanical Engineering, Manufacturing Group, for his
valuable instructions and suggestions throughout this project.



A/Prof Loh Han Tong, Co-supervisor, National University of Singapore,
Department of Mechanical Engineering, Manufacturing Group, for his
continuous suggestions and support.



A/Prof Lu Wen Feng, National University of Singapore, Department of
Mechanical Engineering, Manufacturing Group, for providing numerous ideas
and useful discussions.



Prof Jerry Fuh Ying Hsi, National University of Singapore, Department of
Mechanical Engineering, Manufacturing Group, for his kind concern and
support.



Dr. Ronny Tham Quin Fai and Dr. Ong Fook Rhu, Singapore Polytechnic,
Biomechanics Laboratory, for their kind help and cooperation in this project.



Mr. Huang Wei Hsuan, Ms. Chen Mingqiong, Mr. Wu Shao Rong and Mr.
Kuan Yee Han, Project Team Members, National University of Singapore,
Department of Mechanical Engineering, Manufacturing Group, for their
assistance and contributions in the project.

I wish to thank the Final-year Project students in Singapore Polytechnic who have
been involved in this project for their contributions and efforts in this project. I also
appreciate the members at Centre for Intelligent Products and Manufacturing System
(CIPMAS) laboratory: Zhou Jinxin, Xu Qian, Ng Jinh Hao, Wang Xue, Chang Lei for
their helpful group discussions and ideas, and the staff of the Advanced
Manufacturing Laboratory (AML), Control Laboratory for their support and technical
expertise in overcoming the many difficulties encountered during the course of the

project.
Last but not least, I would like to acknowledge the participation of all the
experimental subjects in this project. I offer my regards and blessings to all of those
who supported me in any respect during the completion of the project.
I


Table of Contents

Table of Contents
Acknowledgements ................................................................................................................... I
Table of Contents.................................................................................................................... II
Summary

......................................................................................................................... IV

List of Tables ......................................................................................................................... VI
List of Figures ...................................................................................................................... VII
List of Abbreviations ............................................................................................................. IX
CHAPTER 1 Introduction ..................................................................................................... 1
1.1

Prolonged sitting........................................................................................................ 1

1.2

Research objectives ................................................................................................... 4

1.3

Organization of the thesis .......................................................................................... 5

CHAPTER 2 Literature review ............................................................................................. 7
2.1

Applications of interface pressure information ......................................................... 7

2.1.1 Interface pressure as indicator of sitting behaviors .................................................. 7
2.1.2 Interface pressure as evaluation measure of supporting surfaces ........................... 11
2.2

Interface pressure measurement techniques ............................................................ 13

2.2.1 Main category of pressure sensors.......................................................................... 13
2.2.2 Major interface pressure measurement devices ...................................................... 15
2.3

Interface pressure analytical methods...................................................................... 17

CHAPTER 3 Interface pressure measurement devices ..................................................... 22
3.1

Background ............................................................................................................. 22

3.2

Evaluation of Piezoresistive sensors ....................................................................... 23

3.2.1 Experimental setup ................................................................................................. 24
3.2.2 Investigation methods ............................................................................................. 26

3.2.3 Results and discussion ............................................................................................ 28
3.3

Characterization of Pressure Mapping System (PMS) ............................................ 31

3.3.1 Selection of PMS .................................................................................................... 33
3.3.2 Experimental setup ................................................................................................. 35
II


Table of Contents

3.3.3 Investigation methods ............................................................................................. 37
3.3.4 Results and discussion ............................................................................................ 44
3.4

Conclusion ............................................................................................................... 46

CHAPTER 4 Methods of interface pressure analysis ........................................................ 49
4.1

Image data preprocessing ........................................................................................ 52

4.1.1 GMM based thresholding ....................................................................................... 53
4.1.2 Neighborhood based method .................................................................................. 57
4.2

Image data registration ............................................................................................ 59

4.2.1 Introduction ............................................................................................................ 59

4.2.2Hausdorff distance ................................................................................................... 62
4.2.3 PSO......................................................................................................................... 63
4.2.4 Results and discussion ............................................................................................ 66
4.3

Static pressure concentration ................................................................................... 72

4.4

Dynamic pressure change ........................................................................................ 74

4.5

Dynamic sitting sway .............................................................................................. 80

CHAPTER 5 Subject interface pressure testing ................................................................ 83
5.1

Objectives ................................................................................................................ 83

5.2

Experimental method............................................................................................... 83

5.2.1 Subjects .................................................................................................................. 83
5.2.2 Experimental setup ................................................................................................. 84
5.2.3 Experimental procedure.......................................................................................... 85
5.3

Results and discussion ............................................................................................. 88

CHAPTER 6 Conclusions and recommendation ............................................................. 101
6.1

Conclusions ........................................................................................................... 101

6.2

Recommendation for future work.......................................................................... 103

References

....................................................................................................................... 104

III


Summary

Summary
Sitting is a common posture in daily lives. It has been extensively studied with
respect to supporting surface, sitting posture, subject groups and other related aspects.
The interface pressure between the human buttock and the supporting surface is an
important metric which has been generally adopted for the evaluation of sittingrelated issues. In order to provide a comprehensive view on the major issues of
interface pressure, a complete process of the specific interface pressure data
acquisition and methods of analysis as well as human testing experiments is presented.
In this project, three kinds of interface pressure measurement sensors,
consisting of Tekscan Flexiforce sensor, Body Pressure Measurement System (BPMS)
and CONFORMat were compared in terms of measurement accuracy, drift and other
sensing characteristics. Based on the comparison, the CONFORMat was selected for

further characterisation. For CONFORMat, the triggering force threshold of crosstalk
interference and inactive sensors were investigated for avoidance of such phenomena.
In addition, the drift properties and measurement accuracy were evaluated and found
to be acceptable. Preliminary sitting tests also showed satisfactory results with regard
to the sensor performance for human subject experiment.
Interface pressure analytical methods were developed for pre-processing of the
pressure patterns to capture certain features of the pressure data. Firstly, a
neighbourhood based thresholding method has been developed and found to be
effective in removing outliers and reconstructing the voids in the pressure pattern.
Secondly for the image registration, a new Particle Swarm Optimization (PSO) based
registration method adopts the Hausdorff distance as indicator of the match between
two pressure patterns. This method was verified to achieve more than 98% success
IV


Summary

rate in pressure pattern registration. The third method concerns pressure concentration
which is harmful in sitting. The static pressure concentration can be identified by a
threshold based method and dynamic pressure change can be recognized by a t-type
test method. For a single-frame pressure pattern, the static pressure concentration is
quantified by a pressure concentration rate whereby the concentrated area is also
segmented. For multi-frame pressure sequence, the dynamic pressure change region
can be identified by applying a t-type test to determine statistically significant changes.
Lastly, a method for plotting the trajectory of centre-of-pressure (COP) and
computing the COP movement range is introduced. COP is an important indicator for
sitting stability and posture change.
For testing of the pressure measurement hardware and the aforementioned
analytical methods, subject testing was conducted. 12 subjects were recruited for three
kinds of sitting: static sitting, side sitting and cross-legged sitting on both hard surface

(HS) and a commercial cushion called ROHO. The results show that the ROHO
cushion is efficient at removing pressure peaks compared with the hard surface. The
study on the dynamic pressure change indicates that side sitting is beneficial for
prolonged sitting as it can greatly reduce the concentrated pressure in the lifted leg
area. When the COP trajectory and movement range of side sitting and cross-leg
sitting were compared, the latter appeared to have a more consistent sitting posture
with similar COP trajectories. Furthermore, cross-leg sitting on hard surface generates
much smaller COP movement range compared to ROHO, which is usually related to
better sitting stability.

V


List of Tables

List of Tables
Table 1.1 Symptoms in prolonged sitting.................................................................................. 2
Table 3.1 Comparison between Flexiforce sensors and FSR sensors[61] ............................... 24
Table 3.2 Comparison between the test results and

sensor specifications of Flexiforce

sensors ..................................................................................................................................... 31
Table 3.3 Comparison of technical specifications of BPMS and CONFORMat .................... 33
Table 3.4 Comparison of results for pressure measurement ................................................... 34
Table 3.5 Comparison of results for area measurement in different points............................. 34
Table 3.6 Actual Mass, Calculated Mass and Percentage Error on CONFORMat ................ 42
Table 3.7 Comparison of results for seating condition with both leg rested ........................... 44
Table 4.1 The major methods developed for interface pressure analysis ................................ 51
Table 4.2 GMM parameter estimation by EM algorithm ........................................................ 55

Table 4.3 Success rate for different Km ................................................................................... 69
Table 4.4 Success rate for pressure pattern registration .......................................................... 71
Table 4.5 Success rate for modified PSO based registration method ...................................... 72
Table 4.6 The COP movement range at four directions .......................................................... 82
Table 5.1 The anthropometric data of the experimental subjects ............................................ 84
Table 5.2 The fc for static sitting on HS and ROHO for three threshold levels....................... 90
Table 5.3 Dynamic pressure change for side sitting on HS and ROHO................................. 94
Table 5.4 Dynamic pressure change for cross-leg sitting on HS and ROHO......................... 95
Table 5.5 COP movement range in side sitting and cross-leg sitting on HS and ROHO ........ 98

VI


List of Figures

List of Figures
Figure 2.1 Ischial tuberosities

[17]

............................................................................................... 7

Figure 2.2 Pressure mapping systems (a)Tekscan BPMS (b)Xsensor Pressure-Mapping Mat
(c) Force Sensing Array (FSA)................................................................................................ 16
Figure 2.3 Hexagonal representation of the six parameters[55] ............................................... 18
Figure 2.4 Pressure Data for all subjects on one of the cushion variants after frequency
analysis [57] ............................................................................................................................... 20
Figure 2.5. The IT region: (a) a typical AB subject; (b)a typical SCI subject sitting in a
controlled posture[14] ................................................................................................................ 21
Figure 3.1 (a) FSR sensors by interlink Electronics, Camarillo, CA, US; (b) Flexiforce

sensors by Tekscan Inc., Boston, MA, US. ............................................................................. 23
Figure 3.2 Schematic illustration of setup for calibration using pneumatic method ............... 25
Figure 3.3 Setup for calibration using pneumatic method....................................................... 25
Figure 3.4 (a) Sensor test on a soft surface; (b) Result of soft surface vs. hard surface.......... 28
Figure 3.5 P-V Relationship for Flexiforce sensor 3 ............................................................... 28
Figure 3.6 1/R-P Relationship for Sensor 1............................................................................. 29
Figure 3.7 Repeatability Test of sensor 3 ................................................................................ 29
Figure 3.8 Hysteresis test for sensor 3..................................................................................... 30
Figure 3.9 Drift test for sensor 8 at P = 30.2kPa ..................................................................... 30
Figure 3.10 Schematic of electronics in pressure measurement mats; (b) Schematic diagram
of measurement area in pressure measurement mats [62] ......................................................... 33
Figure 3.11 Crosstalk interference for the cells in the vertical direction: (a) at the side;( b) in
the center ................................................................................................................................. 38
Figure 3.12 Location of inactive sensor .................................................................................. 39
Figure 3.13 Pressure-Time distribution (a) 60s (b) 180s (c) 300s (d) 600s (e) 1,800s ............ 40
Figure 3.14 Graph of drift analysis for weights from 10kg to 50kg ........................................ 41
Figure 3.15 Graph of Actual Mass vs Calculated Mass .......................................................... 42
Figure 3.16 Pressure distribution for different seating positions (Pattern 1~ 6)...................... 43
Figure 4.1 Original interface pressure pattern with outliers and vacancies ............................. 52
Figure 4.2 Histogram of Figure 4.1. (Red line indicates the visual estimation of mixture
Gaussian distributions) ............................................................................................................ 53
Figure 4.3 GMM estimation of pressure data of Figure 4.1 (CPU time used for EM_GM:
2.97s; Number of iterations: 23) .............................................................................................. 55
Figure 4.4 The processed pressure pattern using T= 48.956 ................................................... 56
Figure 4.5 Schematic of neighborhood of pixel P ................................................................... 57
VII


List of Figures

Figure 4.6 Example of pre-processing result of using the neighborhood based thresholding
method: (a) original image; (b) processed image. ................................................................... 58
Figure 4.7 Study on neighbourhood based thresholding (a) original pressure pattern, (b)
threshold=4, (c) threshold=5, (d) threshold=6, (e) threshold=7. ............................................. 59
Figure 4.8 Image registration: (a) source image A, (b) target image B. .................................. 60
Figure 4.9 Spatial registration method based on a line and a point for interface pressure data
................................................................................................................................................. 61
Figure 4.10 Example of asymmetrical pressure pattern .......................................................... 62
Figure 4.11 Matching results comparison ............................................................................... 68
Figure 4.12 Convergence of PSO based image registration .................................................... 70
Figure 4.13 Three kinds pressure pattern registration ............................................................. 70
Figure 4.14 Modified registration method: (a) the local smallest Hausdorff distance in the 10
subsets (b) an example of improved match ............................................................................. 72
Figure 4.15 Static pressure concentration ............................................................................... 74
Figure 4.16 Dynamic pressure change analysis flow chart ..................................................... 75
Figure 4.17 Smoothing of difference movie............................................................................ 78
Figure 4.18 An example of the complete process and result of the dynamic pressure change
analytical method .................................................................................................................... 79
Figure 4.19 (a) A snapshot of a pressure movie (b) the COP trajectory of the pressure movie
................................................................................................................................................. 82
Figure 5.1 (a) The experiment setup (b) ROHO Quadtro Low Profile Cushion ..................... 85
Figure 5.2 The central sitting posture ...................................................................................... 86
Figure 5.3 The data acquisition parameters for all the three session of pressure record ......... 87
Figure 5.4 The original pressure pattern and preprocessed pressure pattern ........................... 89
Figure 5.5 3D display of the typical pressure distribution pattern of sitting on (a)hard surface
(b)ROHO cushion.................................................................................................................... 90
Figure 5.6 Dynamic pressure change analysis: Subject s07. .................................................. 93
Figure 5.7 The typical COP trajectory patterns for side sitting and cross-leg sitting on HS and
ROHO (s07) ............................................................................................................................ 96
Figure 5.8 COP trajectory of side sitting and cross-leg sitting on hard surface (s10) ............. 97

Figure 5.9 The comparison of the range of COP trajectory: 1) side sitting on HS; 2) Cross-leg
sitting on HS; 3) side sitting on ROHO; 4) Cross-leg sitting on ROHO. ................................ 98

VIII


List of Abbreviations

List of Abbreviations
AI

Artificial Intelligence

BH-FDR

Benjamini and Hochberg False discovery rate control

BPMS

Tekscan Body Pressure Measurement System

COP

Centre-of-pressure

EM

Expectation-Maximization

FDR

False Discovery Rate

FSA

Force Sensing Array

FSR

Force Sensing Resistor

GA

Genetic Algorithm

GMM

Gaussian Mixture Modeling

HS

Hard surface

IT

Ischial tuborosities

LASR

Longitudinal Analysis with self-Registration

PCA

Principal component analysis

PMS

Pressure Mapping System

PSO

Particle Swarm Optimization

PU

Pressure ulcers

ROHO

Tekscan ROHO cushion

ROI

Region of Interest

SCI

Spinal Cord Injury

SRLP

Spatial Registration method based on a Line and a Point

SVD

Singular Value Decomposition

TPM

Talley Pressure Monitor III

IX


Development of an approach for interface pressure measurement and analysis for study of sitting

CHAPTER 1

Introduction

1.1 Prolonged sitting
Modern living increases the tendency to have a more sedentary lifestyle that involves
sitting. In particular, as the use of computers and computing technologies in the
workplace increases, there has been a significant increase in the proportion of seated
occupations in recent decades [1]. Published estimates have indicated that almost 75%
of work in industrial countries is performed while seated [2]. From a biomechanical
perspective, sitting is an easy and more stable posture with low-energy
consumption[3], lower centre of mass and larger base of support [4]. However,
prolonged sitting during daily activities can develop stress in muscles of the back,
buttocks, and legs. Various problems related to prolonged sitting have long been

reported and studied. As summarized in Error! Not a valid bookmark self-reference.,
discomfort, muscle fatigue, inhibited blood flow and many chronic problems, such as
neck pain, low back pain are commonly encountered by office workers who spend
large portion of time sitting. For example, low back pain is a major health problem
within industrialized populations. According to a survey published in 2000, almost
half of the adult population of the U.K. (49%) report low back pain lasting for at least
24 hours at some time during the year [5]. Active prevention of these syndromes is a
priority.
In addition, sitting is also among the most fundamental activities of daily living for
the disabled or aged who is wheelchair or bed bounded. For these people who have
limited mobility and impaired sensation, prolonged sitting will be highly risky and
harmful for them. This degenerates further into problems of pressure ulcers, spasticity,
1


Development of an approach for interface pressure measurement and analysis for study of sitting

instability and even deterioration in some physical functions, as summarized in Table
1.1.
Table 1.1 Symptoms in prolonged sitting
Healthy People

Disabled/Aged People

- Discomfort;

- Pressure ulcers;

- Muscle fatigue;

- Spasticity (Contraction of muscle groups);

- Inhibited blood flow ;

- Instability;

- Chronic occupational disease:

- Deterioration in physical functions…

- Neck pain, low back pain…

Pressure ulcers (PU), also known as a decubitus ulcer, are a serious problem due to its
prevalence and significant harm. The prevalence of pressure ulcers is 18.1% in
European standard and academic hospitals[6], 23% for hospitals and 25% for nursing
homes in the Netherlands[7]. Depending on the severity of the ulcers, complications
could range from delayed healings to mortality[8]. In particular, treatment of pressure
ulcers is not only painful but also time consuming and costly [7]. The factors causing
pressure ulcers are complicated, and according to previous research, they mainly
include the pressure under bony prominences, shear forces, temperature, moisture,
nutrition, seating position and daily life routine [9-11]. Although clinical and research
evidence in this area is inconclusive and conflicting, excessive pressure between
human buttock and seating surface is generally recognized as the principal cause of
the occurrence of pressure ulcer[8]. Higher interface pressure measurements are
associated with a higher incidence of sitting-acquired pressure ulcers for high-risk
elderly people who use wheelchairs[9].

2

Development of an approach for interface pressure measurement and analysis for study of sitting

External sitting environment, including the ambient environment, supporting surface,
and occupant’s internal anatomy structure and even emotions can affect the
occupant’s perception of sitting. Posture, tissue deformity and pressure on the
buttocks at the seating interface are the main factors used in clinical and rehabilitative
management of individuals requiring wheelchairs and specialized seating[12]. As the
pressure between the human buttock and the supporting surface, which is usually
referred as interface pressure, can objectively and quantitatively characterize the
supporting surface and its interaction with the subject, it has been consistently
employed in the study of sitting-related issues. The quantitative and objective
collection of interface pressure data have been identified and corroborated repeatedly
as an appropriate metric for assessing the impact of seating related variables, such as
posture, seat construction and structural support of the body. For example, interface
pressure measurement is suggested as the primary task in the research of pressure
ulcers [2, 13-16].
Considering the important role of interface pressure, numerous research techniques
and devices have been developed in an attempt to quantify the interface pressure.
However, the selection of interface pressure measurement devices based on study
requirements is the first challenge. After accurate interface pressure distribution data
is captured, the next task is efficient analysis of the pressure data to get pressure
features. However, techniques for the quantitative analysis of interface pressure data
have not kept pace with the development of the measurement sensors and instruments.
Advanced analytical methods have been reported for specific applications in research
studies, but none of these can completely fulfil the project requirements and thus need
further improvements.
3


Development of an approach for interface pressure measurement and analysis for study of sitting

This chapter provides a brief overview of the common risks encountered in
prolonged sitting and general application of interface pressure in prolonged sitting
study. The motivation of the thesis is presented, followed by the detailed description
of the research scope.

1.2 Research objectives
Measurement and analysis of interface pressure are major tasks in the study of sittingrelated issues. This project aims to find a suitable interface pressure measurement
device as for data acquisition. Furthermore, as current study in interface pressure data
analysis is still limited, the major objective of this thesis is also to develop a set of
new interface pressure analytical methods by integrating advanced data mining
techniques and pattern recognition tools. In addition, the effectiveness of the newly
developed analytical methods will be verified by preliminary subject testing
experimental data. The main objectives of this project are:


Selection and evaluation of interface pressure measurement devices

This project will identify a suitable interface pressure measurement device based on
comparison and testing of different devices. Systematic calibration and evaluation of
the selected devices will be conducted to achieve desired accuracy for project.


Preprocessing of interface pressure data

Major preprocessing tasks include removal of outliers and reconstruction of vacant
sensing information to get constant pressure information.


Static interface pressure analytical methods

4


Development of an approach for interface pressure measurement and analysis for study of sitting

For single frame interface pressure distribution pattern, also referred to as static
interface pressure data in this thesis, analytical methods are developed to find the
pressure concentration area. The outcome results are important quantitative indicators
of the risk of buttock tissue injury of the seated subjects.


Dynamic interface pressure analytical methods

When a subject sits for a long time, longitudinal interface pressure data can be
recorded in the form of successive frames of pressure patterns (named as “movie” in
this thesis). Significant change in the area pressure during the entire sitting time will
be identified by comparison with a baseline measurement. This information will be
helpful for clinicians to identify the changes of the subject’s sitting conditions.


COP trajectory and movement range

Additionally, the sway information of the occupant will be characterized by analyzing
the trajectory of the occupant’s centre-of-pressure (COP). The range of COP
movement is a quantitative indicator related to sitting stability.


Subject testing experiment

Intended subject testing experiment will be conducted to evaluate the effectiveness of
the interface pressure analytical methods. Other objectives of the subject testing
experiment also include comparing the different supporting surface and characterize
different sitting modes.

1.3 Organization of the thesis
The thesis is organized as follows:

5


Development of an approach for interface pressure measurement and analysis for study of sitting



Chapter 2 reviews the major interface pressure applications, measurement
techniques and analytical methods that have been reported recently. The
progress and challenges in this area are summarized.



Chapter 3 presents the testing and comparison results of two interface pressure
devices, flexiforce sensor and pressure mapping system (PMS). Detailed
calibration and characterization of the PMS performance are given.



Chapter 4 compares the two categories of interface pressure analytical
techniques: static interface pressure analytical methods and dynamic analytical
methods. The major computational technique and output results are

demonstrated.



Chapter 5 presents the experimental setup and results of study of the subject
testing data. The results are computed using the methods introduced in
Chapter 4. Further conclusions from the experiment are discussed.



Chapter 6 concludes the work and puts forth recommendations and future
work.

6


Development of an approach for interface pressure measurement and analysis for study of sitting

CHAPTER 2

Literature review

2.1 Applications of interface pressure information
Interface pressure is defined as the pressure distribution between the human buttock
and the supporting surface in sitting. It has been extensively adopted to evaluate the
occupant’s sitting behaviors and properties of the supporting surface in both clinical
and academic studies.

2.1.1 Interface pressure as indicator of sitting behaviors
Sitting is a body position in which the body weight is transferred to a supporting area,

mainly by the ischial tuborosities (IT, sitting bones) of the pelvis and their
surrounding soft tissues, as shown in Figure 2.1.

By investigating the interface

pressure between the human buttock and supporting surface, researchers can get
important information about subject’s sitting behaviors.

Figure 2.1 Ischial tuberosities[17]

7


Development of an approach for interface pressure measurement and analysis for study of sitting

Posture is one of the most important factors in the study of sitting-related issues.
Medical and ergonomic field studies indicate that bad sitting postures are sometimes
accompanied by pains in tissues and other serious complications for more vulnerable
subjects. Extensive studies have been done to evaluate different sitting postures using
the interface pressure data. In a study evaluating different postures for both healthy
and Spinal Cord Injury(SCI) subjects, it was found that the maximum pressures can
be reduced by up to 12% by postural changes[18]. This conclusion confirmed the
general knowledge that some postures have better pressure relieving capacities.
Furthermore, according to Hobson, the posture in which the lowest maximum
pressure was measured was the sitting-back posture with the lower legs on a rest[19].
Makhson’s research group proposed a partially removed ischial support posture, and
found that the concentrated interface pressure observed around the ischia in normal
posture was significantly repositioned to the thighs in the new posture[20].
Furthermore, sitting posture can significantly affect pelvic orientation and ischial
pressure[21]. There are also numerous studies focused on the sitting postures of

different subject groups, such as drivers[15], office workers, children[22] and some
other subjects

which also taking interface pressure as an objective evaluation

measurements.
Body posture directly influences seating load and proper postural change is therefore
essential. In prolonged sitting, the repositioning of the high-risk patient with limited
mobility and sensation is a regular task for the nurse or caregiver. Essentially, the
repositioning attempts to shift the pressure concentration from one area to another to
avoid prolonged stress concentration. Aimed at investigating the reposition ability and
the intervention methods efficiency, interface pressure is usually measured and
8


Development of an approach for interface pressure measurement and analysis for study of sitting

evaluated. Geffen et al described a mechanism for postural adjustments which
includes the seat inclination, pelvis rotation and chair recline and concluded that a
combination of independent pelvis rotation and seat inclination is effective to regulate
the sacral interface pressure in healthy subjects[23]. In addition, as pelvis alignment
directly affects body posture and buttock load, a passive motion technique, decoupled
pelvis rotation was evaluated and significant relations were found between pelvis
rotation and most quantities of interface pressure. Therefore decoupled pelvis rotation
was suggested to be an effective technique to regulate buttock load in able-bodied
individuals[24]. However, the effectiveness of these techniques on disabled subjects
for clinical applications still needs further explorations. It was also found that the
maximum pressure depends on the angle of pelvis rotation, which confirmed the
pressure relief effects of the repositioning[25]. Other than rotation of pelvis, postural
change can also be evaluated by measuring the movement of ischial tuborosities. Peak

pressure locations did not coincide exactly with the ischial tuberosities during
wheelchair propulsion[26]. Furthermore, when subjects were required to shift
postures, the frequency of shifting is important. Changing the sitting load at least
every 8 minutes is recommended for wheelchair users by Reenalda, et al[17]. This
can be used as a reference for preventing pressure ulcers.
Sitting comfort is a major concern for drivers and other members of the work force
who are exposed to extended periods of sitting and its associated side effects.
Research on the effects of pressure distribution have shown that compression, shear
pressures, or both, that develop at the human-seat interface are the main causes of
seating discomfort[12]. More specifically, several pressure variables were identified
as more effective to assess sitting comfort and improve seat quality [27-28]. However,
9


Development of an approach for interface pressure measurement and analysis for study of sitting

for wheelchair users, the cushions that they feel most comfortable were not
necessarily those providing the lowest interface pressures[29]. This result calls for
deeper study of other interface pressure features rather than simple magnitude of
interface pressure. Earlier study on indirect measurement of sitting discomfort by
tracking the COP showed promising results as COP can well characterize the
subject’s in-chair movement, which was related to sitting discomfort[30]. Basically,
customers’ feeling of comfort is vital for the purchase[31]. Thus evaluation of
subjective feeling by objective measurement of interface pressure shows potential in
both the cost feasibility and reliability considerations; however, further systematic
study is required as results about the comfort and interface pressure is still
inconclusive and even conflicting.
The relationship between interface pressure and the sitting subject’s anthropometrical
and anatomical information has also been explored. Al-Eisa found that the leg length
discrepency group had a much larger variance in pressure than the symmetrical

group[32]. Spinal Cord Injury(SCI) subjects are usually more prone to pressure
ulcers,and it can be well explained by the observation that the weight bearing on the
IT for the SCI is distributed on half the surface in comparison with the abled group or
the powered wheelchair users groups[14]. The findings of this study provide insights
concerning pressure distribution in sitting for the paraplegic as compared to the ablebodied. In addition, gender difference also affects the pressure distribution due to the
different body profile and the skeletal shape. Thus gender-dependent treatment
modalities should be implemented in seating based on the finding that males and
females may be exposed to different loading patterns during prolonged sitting and
may experience different pain generating pathways[33]. Currently, there is very little
10


Development of an approach for interface pressure measurement and analysis for study of sitting

information in the scientific literature regarding the identification of the features of
the seated subjects. This may be attributed to the fact that present interface pressure
analysis techniques are limited in ability to accquire more useful information, which
will be discussed more in section 2.3.

2.1.2 Interface pressure as evaluation measure of supporting surfaces
As discussed above, interface pressure can be used to evaluate the subject’s sitting
posture and comfort, which are important aspects in the evaluation of the supporting
surface; therefore it has been generally used as an objective method to assess cushion
and seat design, yet existing evidence regarding its efficacy is mixed.
Presently, commercial cushioning products for pressure ulcer prevention are being
evaluated for their protective effect exclusively based on interface pressures.
Laboratory developed cushioning products are more versatile and complicated.
However there does not exist a “golden standerd” for testing[34]. It means there is not
a generally accepted evaluation criteria for the cushion products in the form of
interface pressure data. Lower interface pressure, more even pressure distribution and

larger contact area are most frequently cited in literature. This can be interpreted that
larger contact surface can effectively reduce the load and that the compression to the
buttock tissue. When compared to Polyurethane foam cushion, the ROHO cushion,
which is a multi-cell type air cushion, was shown to be more efficient in
compensating the adverse effects of sitting posture on pressure distribution[21, 35].
However, due to different experimental conditions, it is impossible to make a simple
conclusion about the optimal cushion. Among the popular wheelchair seat cushions, a
dual-compartment air cushion was identified as the best for the largest contact
surface[36]. In evaluting the

pressure relieving effect of the four seat cushions
11


Development of an approach for interface pressure measurement and analysis for study of sitting

designed for incontinent patients, a thick air cushion has the lowest maximum
pressure when slouching or sliding down[19]. In addition, interface pressure also play
an important role in design and optimization of new cushion products[37]. Brienza
used interface pressure and stiffness to optimize the surface shape of a custom
contoured form seat cushion in the hope of minimizing the tissure deformation.
Results show improved effectiveness of the optimized cushion versus flat foam
cushions[38]. Goetz did a study to examine two alternating air cell mattresses used for
pressure ulcer prevention and treatment in a SCI population. Interface pressure
characteristics of the two mattresses were very different, and neither mattress retained
performance in the 45-degree position[39]. However, some researchers argued that
cushion comfort is not related to interface pressure[29], as discussed in previous
section.
Design and evaluation of chair or vehicle seat also involves study of the interface
pressure. Chair design differences had the greatest effect on seat pan interface

pressure, compared to participant effects, and lastly postural treatments[40].
Furthermore, the vehicle vibration was investigated via monitoring the interface
pressure change. Study results showed that the maximum variations in the ischium
pressure and the effective contact area on a soft seat occur near the resonant frequency
of the coupled human–seat system (2.5–3.0 Hz)[41]. Compared with flat supporting
surfaces, the contact area was greatest on the exercise ball[42]. The results of this
study suggested that sitting on a dynamic, unstable seat surface appears to spread out
the contact area.
Interface pressure has also been used in evaluation of rehabilitation products and
clinical interventions. Application of a thoraco-lumbar-sacral orthosis in a child with
12


Development of an approach for interface pressure measurement and analysis for study of sitting

scoliosis significantly reduced the spinal curvature and interface sitting pressure[43].
A mechanical automated dynamic pressure relief system was compared with a
standerd wheelchair for pressure relieving capacity. In the off-loading configuration,
concentrated interface pressure during the normal sitting configuration was
significantly diminished[44]. Additionally, sacral anterior root stimulator implants
was tested to prevent ischial pressure ulcers in the SCI population. Results indicated
that sacral nerve root stimulation induced sufficient gluteus maximus contraction to
significantly change subjects' ischial pressures during sitting[45]. This finding is
consitent with the experiment done by Liu, et al[46].

2.2 Interface pressure measurement techniques
There is a need in the automotive and rehabilitative industries to obtain objective
measures for sitting condition monitoring and seat evaluation. Interface pressure
measurement is usually taken as a rapid, easily quantifiable data which would indicate
the areas at risk of tissure damage. In this section, several major interface pressure

measurement techniques are reviewed.

2.2.1 Main category of pressure sensors
The main types of sensors to measure seat-buttock interface pressure used and
reported are generally classified into these categories: resistive sensors, capacitive
sensors, electro-pneumatic sensors and constant pneumatic sensors[47].
Sensors with force sensitive resistive or capacitive materials can be further
categorized as electronic sensors.
Resistive sensors

13


Development of an approach for interface pressure measurement and analysis for study of sitting

The working principle of resistive sensor is the variation of resistance of a
piezoresistive layer when a force is applied [48]. The most common piezoresistive
technology utilises two thin flexible polymer sheets with conductive material applied
to either one sheet or both sheets to achieve a planar wiring configuration or a more
flexible wiring configuration[48]. The resistive layer consists of strain gauges or
force-sensing resistors that maps the applied force and translates it into a pressure
reading. The pressure reading remains constant as long as the pressure applied does
not change.
Capacitive sensors
Capacitive sensors, as named, make use of capacitors when measuring pressure. Most
capacitors consist of two metal plates with opposite electrical charges. The amount of
electrical charges stored by the capacitor depends on the size of metal plates, and the
distance between the plates since E stored 

where

1
A
CV 2 and C 
2
d

V = voltage across the capacitor
ε = permittivity of the dielectric
A = area of the plates of the capacitor
d = distance between the plates

The change in distance between the plates causes a change in capacitance and is used
to determine the pressure applied.
Most suppliers prefer piezoresistive sensors to capacitive sensors because
piezoresistive sensors are fast, relatively simple and have a low sensitivity. However,
some experts in the field favour capacitive sensors due to the disadvantages of
resistive sensors (non-linearity, temperature and humidity dependent and poor
stability)[48].
14


Development of an approach for interface pressure measurement and analysis for study of sitting

Electronic sensors are most commonly used as they are readily available.
Commercially available Force Sensing Array pad (FSA) by Vista Medical and Body
Pressure Mapping System (BPMS) by Tekscan make use of resistive sensor
technology while Pliance Sensors and Xsensor sensors make use of capacitive sensor
technology.
Electro-pneumatic sensors

Electro-pneumatic sensors consist of a flexible and inflatable sac inside which
electrical contact strips are placed diagonally. The sensor is positioned between the
patients’ bottoms and the supporting materials at the site of interest. Air is slowly
pumped into the sensor and when internal and external pressures are in equilibrium,
the electrical contact between both strips breaks. Pressure recorded at that moment is
considered to be the interface pressure[47].
Constant pneumatic sensors
Pneumatic sensors consist of air cells connected to a high pressure pump with
pressure exceeding that applied to the sensor. The working principles of the sensors
are as follows: the sensor is inflated by the air pump. The volume of air in the sensor
increases suddenly as the inflation pressure rises above the pressure applied, resulting
in a rapid drop in the rate of pressure increase. The pressure in the air pump at that
moment is recorded as the interface pressure.

2.2.2 Major interface pressure measurement devices
Pressure mapping systems such as the Tekscan “Big-Mat”, Tekscan BPMS, Xsensor
pressure-mapping mats and Force Sensing Array pad (FSA), by Vista Medical as
shown in Figure 2.2, are commonly used for interface pressure measurement because
they are very thin (the thickest of which is 0.36mm) and flexible. These pressure mats
15